Method for making moisture insensitive zinc sulfide based luminescent materials

ABSTRACT

A process is disclosed for coating phosphors with hydrolyzed alkylaluminum. The hydrolyzed alkylaluminum coating renders the phosphors insensitive to atmospheric moisture. the coating process involves vaporizing an aluminum-containing precursor such as trimethylaluminum or triethylaluminum in an inert gas stream and passing this through a fluidized bed containing the phosphor particles. Water vapor is also passed through the fluidized bed and the water and aluminum precursor react on the surface of the phosphor particles to form hydrolyzed trimethylaluminum or other alkylaluminum. The hydrolyzed trimethylaluminum or other alkylaluminum phosphors are particularly useful in electroluminescent lamps.

BACKGROUND OF THE INVENTION

The present invention describes a process for coating finely dividedmaterial with hydrolyzed aluminum. More specifically electroluminescentphosphors coated with hydrolyzed trimethyl aluminum have been producedwhich are almost completely insensitive to atmospheric moisture.

Zinc sulfide-based phosphors, typically doped with copper, may bestimulated to emit visible light by the absorption of electrical energyin a so-called electroluminescent lamp where a layer of the phosphor issandwiched between a front transparent electrode and a backnon-transparent electrode with a layer of insulating material (typicallybarium titanate) sandwiched between the phosphor layer and the backelectrode. However, the luminescent efficiency of the phosphor degradesmuch more rapidly if the phosphor is exposed to a moisture-containingatmosphere than if it is exposed to a very dry atmosphere. The inventionwhich is the subject of this disclosure was motivated by a desire toreduce the moisture sensitivity of such zinc sulfide-based phosphors.The selected approach was to form a thin yet continuous coating ofhydrolyzed trimethyl aluminum (TMA) upon the surfaces of the zincsulfide particles, thereby protecting them from the effects ofatmospheric moisture.

The hydrolyzed TMA coatings are formed via chemical vapor depositionwith the phosphor particles suspended within a gas-fluidized bed.Hydrolyzed TMA, presumably consisting mainly of relatively amorphousaluminum hydroxide, was selected as a coating material principallybecause it can be formed at relatively low temperatures by the reactionof gaseous TMA with gaseous water molecules without the use of oxygen orany other coreactant. In this way, the coatings can be formed underconditions that are least likely to modify the surface chemicalcomposition of the relatively reactive zinc sulfide based phosphor.

SUMMARY OF THE INVENTION

The present invention describes a process for forming a coating ofhydrolyzed trimethyl aluminum on the outer surfaces of phosphorparticles. Trimethyl aluminum is vaporized in an inert carrier gas andwater is likewise vaporized in an inert carrier gas. The two carrier gasstreams are passed through a fluidized bed of phosphor particles whereinthe trimethyl aluminum reacts with the water on the phosphor particlesurfaces to form a coating of hydrolyzed trimethyl aluminum. The coatedphosphor shows an extreme insensitivity to atmospheric moisture.

In another aspect of the invention phosphor powder particles are coatedwith hydrolyzed trimethyl aluminum and used in electroluminescent lamps.The coated phosphor is sandwiched between a transparent electrode and asecond electrode. The manufactured electroluminescent lamp is resistantto atmospheric moisture.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic drawing of the apparatus used to carry out thecoating reactions;

FIG. 2 shows unpackaged EL lamp data for hydrolyzed TMA coated phosphorwith an aluminum content equivalent to 1.0 w/o Al(OH)₃ and uncoatedphosphor;

FIG. 3 shows unpackaged EL lamp data for hydrolyzed TMA coated phosphorwith an aluminum content equivalent to 2.0 w/o Al(OH)₃ and uncoatedphosphor;

FIG. 4 shows unpackaged EL lamp data for hydrolyzed TMA coated phosphorwith an aluminum content equivalent to 3.0 w/o Al(OH)₃ and uncoatedphosphor;

FIG. 5 shows unpackaged EL lamp data for hydrolyzed TMA coated phosphorwith an aluminum content equivalent to 3.5 w/o Al(OH)₃ and uncoatedphosphor;

FIG. 6 shows packaged EL lamp data for hydrolyzed TMA coated phosphorwith an aluminum content equivalent to 1.0 w/o Al(OH)₃ and uncoatedphosphor;

FIG. 7 shows packaged EL lamp data for hydrolyzed TMA coated phosphorwith an aluminum content equivalent to 2.0 w/o Al(OH)₃ and uncoatedphosphor;

FIG. 8 shows packaged EL lamp data for hydrolyzed TMA coated phosphorwith an aluminum content equivalent to 3.0 w/o Al(OH)₃ and uncoatedphosphor;

FIG. 9 shows packaged versus unpackaged EL lamp data for hydrolyzed TMAcoated phosphor with an aluminum content equivalent to 3.0 w/o Al(OH)₃ ;

FIG. 10 shows packaged versus unpackaged EL lamp data for uncoatedphosphor;

FIG. 11 shows plastic lamp unpackaged to packaged brightness ratios forhydrolyzed TMA coated phosphor with an aluminum content equivalent to 3w/o Al(OH)₃ and uncoated phosphor;

FIG. 12 shows Aclar packaged EL lamp data for hydrolyzed TMA coatedphosphor with an aluminum content equivalent to 6.1 w/o Al(OH)₃ anduncoated phosphor;

FIG. 13 shows polyester packaged EL lamp data for hydrolyzed TMA coatedphosphor with an aluminum content equivalent to 6.1 w/o Al(OH)₃ anduncoated phosphor;

FIG. 14 compares polyester packaged and Aclar packaged EL lamps usinguncoated phosphor;

FIG. 15 compares polyester packaged and Aclar packaged EL lamps usinghydrolyzed TMA coated phosphor with an aluminum content equivalent to6.1 w/o Al(OH)₃ ;

FIG. 16 compares EL lamp data for plastic lamps packaged in Aclar usingdifferent lots of phosphor coated with hydrolyzed TMA;

FIG. 17 compares EL lamp data for plastic lamps packaged in polyesterusing different lots of phosphor;

FIG. 18 compares EL lamp data for plastic lamps packaged in polyesterusing uncoated phosphor of greater than 325 mesh, uncoated phosphor ofless than 325 mesh and hydrolyzed TMA coated phosphor of less than 325mesh;

FIG. 19 compares EL lamp data for plastic lamps packaged in Aclar usinguncoated phosphor of greater than 325 mesh, uncoated phosphor of lessthan 325 mesh and hydrolyzed TMA coated phosphor of less than 325 mesh;

FIG. 20 compares EL lamp data for polyester packaged hydrolyzed TMAcoated phosphor with an aluminum content equivalent to 6.51% Al(OH)₃ anduncoated phosphor;

FIG. 21 compares EL lamp data for polyester packaged TMA coatedphosphors with an aluminum content equivalent to 5.7% and 6.9% Al(OH)₃and uncoated phosphor;

FIG. 22 compares EL lamp data for Aclar packaged uncoated phosphor andpolyester packaged hydrolyzed TMA coated phosphor;

FIG. 23 compares EL lamp data for Aclar packaged uncoated phosphor andpolyester packaged hydrolyzed TMA coated phosphor;

FIG. 24 compares EL lamp data for Aclar packaged washed uncoatedphosphor and polyester packaged washed hydrolyzed TMA coated phosphor;

FIG. 25 shows thermogravimetric data for an uncoated phosphor; and

FIG. 26 shows thermogravimetric data for a hydrolyzed TMA coatedphosphor.

For a better understanding of the present invention together with otherobjects, advantages and capabilities thereof, reference is made to thefollowing disclosure and appended claims in connection with the abovedescribed drawing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The hydrolyzed TMA coatings of the present invention where applied via achemical vapor deposition (CVD) technique. FIG. 1 shows a schematic ofthe system used to carry out the CVD coatings.

Trimethyl aluminum (Al(CH₃)₃) was used as the organo-metallic coatingprecursor. Prior to entering the fluidized bed 16, part or all of theinert fluidizing gas or carrier gas was passed through a bubbler 12filled with liquid TMA heated to a temperature high enough to produce aTMA equilibrium vapor pressure sufficient to yield the desired coatingrate (typically between 25° and 35° C). The carrier gas flow wascontrolled by valves 54 and 55. The inert fluidizing gas was nitrogen.However, argon can also be used. The portion of the fluidizing gas line13 located between the bubbler outlet and the fluidized bed inlet 40 washeated by a heating tape 30 to a temperature a few degrees above the TMAbubbler temperature in order to prevent condensation of the TMA withinthe gas lines. The fluidized bed itself was heated to a temperature ofbetween 150° and 250° C. by furnace 20 during the coating process.

Once within the fluidized bed, the TMA is hydrolyzed by reaction withgaseous water vapor which is transported into the reactor via a secondstream of inert gas such as nitrogen or argon through line 23. Thisinert gas stream is passed through a water-filled bubbler 22 beforeentering the fluidized bed via a hollow tube which terminates at a point19 above but not too far from the fluidized bed gas distributor 33. Thewater bubbler temperature and the flow rate of the water-containinginert gas stream are adjusted so as to deliver a quantity of water tothe fluidized-bed reactor 16 sufficient to react completely with all ofthe TMA molecules entering the reactor via the fluidizing gas stream. Itis best to have a relatively large excess of water present within thereactor so as to ensure that all of the TMA molecules will be completelyhydrolyzed. A five to fifteen fold excess is desirable although thereaction will work with just a three fold excess. The reaction isallowed to proceed until the desired quantity of hydrolyzed TMA coatinghas been produced.

Once the desired thickness of the hydrolyzed TMA coating has formed onthe phosphor particles, the reaction is stopped, the fluidized bed isallowed to cool in an inert gas stream and the coated phosphor particlesare removed from the tubular reactor.

When the hydrolyzed TMA-coated ZnS phosphors are examined by X-rayphotoelectron spectroscopy, the virtual absence of emitted electronsoriginating from Zn, S, or Cu atoms indicates that the hydrolyzed TMAcoatings are continuous, i.e. completely covering the surface of eachphosphor particle.

Sylvania Type 723 ZnS:Cu EL phosphor was used in the Examples 1-6. Theuncoated and hydrolyzed TMA coated phosphors were thoroughly dispersedin a polymeric organic binder (a mixture of cyanoethyl cellulose andcyanoethyl sucrose). Subsequently, thin layers of the uncoated andcoated phosphor dispersions were formed upon the surfaces of transparentsheets of glass or Mylar that had first been coated with thintransparent indium-tin oxide (ITO) which serves as the front transparentelectrode for the EL lamp. A layer of insulating barium titanate,dispersed in the same polymeric organic binder, was subsequently formedon top of each phosphor layer. Finally, layers of conductive carbon toserve as the rear lamp electrode, were formed upon the surfaces of thebarium titanate layers. Copper mesh current collectors were attached toeach electrode. In some cases, the finished lamps were sealed betweentwo sheets of Aclar (a special fluorohalocarbon-based, water impermeabletransparent plastic manufactured by Allied Chemical Corp.) usingconventional laminating heat sealing equipment.

All weight percents of the hydrolyzed TMA coatings are based on themeasured aluminum content which is converted to an equivalent weightpercent of aluminum hydroxide (Al(OH)₃). In the discussion that follows,the coating is not pure aluminum hydroxide.

EXAMPLE 1

Four 300 gm quantities of the copper-doped zinc sulfideelectroluminescent phosphor (lot 361S of Type 723 phosphor availablefrom Sylvania) were coated using the reaction conditions listed inTable 1. Samples 320-90, 327-90, 403-90 and 416-90, were coated for 11/3 hours, 2 2/3 hours, 5 1/3 hours, and 5 1/3 hours, respectively.Sample 320-90 was prepared using a 250° C. reaction temperature, while a200° C. temperature was used to prepare the samples 327-90 and 403-90.Sample 416-90 was prepared using a 123° C. coating reaction temperature.The H₂ O bubbler and TMA bubbler temperatures are shown in Table 1.

                                      TABLE 1                                     __________________________________________________________________________    Reaction Condition Summary                                                                            H.sub.2 O                                             Sample                                                                            Phosphor Lot/                                                                         Reaction                                                                           Equivalent                                                                           Bubbler                                                                            N.sub.2 Flow Rate                                                                       TMA Bubbler                                                                           N.sub.2 Flow                                                                            Reaction             No. Sample Weight                                                                         Temp.                                                                              Al(OH).sub.3 w/o                                                                     Temp.                                                                              Thru H.sub.2 O Bubbler                                                                  Temp.   Thru TMA                                                                                Timeler              __________________________________________________________________________    320-90                                                                            361S/300 gm                                                                           250° C.                                                                     1.01   69° C.                                                                      1 1/min   30° C.                                                                         0.5  1/min                                                                              11/3 hr              327-90                                                                            361S/300 gm                                                                           200° C.                                                                     1.89   68° C.                                                                      1 1/min   30° C.                                                                         0.5  1/min                                                                              22/3 hr              403-90                                                                            361S/300 gm                                                                           200° C.                                                                     2.96   67° C.                                                                      1 1/min   30° C.                                                                         0.5  1/min                                                                              51/3 hr              416-90                                                                            398S/300 gm                                                                           123° C.                                                                     3.53   57° C.                                                                      1 1/min   30° C.                                                                         0.25 1/min*                                                                             51/3 hr              508-90                                                                            361S/300 gm                                                                           200° C.                                                                     6.1    70° C.                                                                      1 1/min   30° C.                                                                         0.5  1/min                                                                              51/3 hr              514-90                                                                            398S/300 gm                                                                           200° C.                                                                     5.8    70° C.                                                                      1 1/min   30° C.                                                                         0.5  1/min                                                                              51/3 hr              521-90                                                                            361S/300 gm                                                                           200° C.                                                                     5.4    70° C.                                                                      1 1/min   30° C.                                                                         0.5  1/min                                                                              51/3 hr              612-90                                                                            361S/300 gm                                                                           200° C.                                                                            70° C.                                                                      1 1/min   30° C.                                                                         0.5  1/min                                                                              51/3 hr              __________________________________________________________________________     *Diluted with an additional 0.25 1/min flow of purified nitrogen         

Electroluminescent lamps were subsequently fabricated with ITO-coatedplate glass serving as the transparent support/transparent frontelectrode. Additional lamps were similarly fabricated using the virgin(uncoated) phosphor, rather than one of the coated phosphor samples. Allof these lamps, without being packaged or in any way protected fromambient laboratory temperature and humidity, were continuously drivenusing a 120 V a.c. power supply operated at 400 Hz. At least two lampsmade from each coated phosphor were tested against a lamp made from theuncoated reference material. The lamp brightnesses were recorded as afunction of time using a calibrated photodiode detector. The maximumbrightnesses measured with the uncoated and coated-phosphor lamps, theratio of the maximum brightnesses (coated versus uncoated), and theratio of the half-life values obtained with the uncoated and coatedphosphor-containing lamps are all listed in Table 2 for each of thecoated-phosphor samples (320-90, 327-90, 403-90 and 416-90). Thehalf-life of a lamp is the time required for the measured brightness todrop to exactly half of its initial maximum value.

As shown in Table 2, the maximum brightnesses measured with the lampscontaining the four coated materials were all within 10% of thatmeasured with the corresponding uncoated-phosphor control lamps. Also,the half-lives of the coated phosphor-containing lamps were all morethan an order of magnitude greater than were those of the correspondinglamps fabricated using the uncoated phosphor. Moreover, the greater themass of the coating i.e., the greater the coating thickness, the largerwas the half-life relative to that of the corresponding control lamp.

The measured brightness versus time data obtained with the lampscontaining coated phosphor number 320-90, along with the data obtainedwith the corresponding uncoated-phosphor control lamp are plotted inFIG. 2. The corresponding data obtained with the lamps fabricated fromsample number 327-90 and its corresponding control lamp are similarlycompared in FIG. 3. The data for sample 403-90 are likewise compared inFIG. 4. FIG. 5 shows the data for lamps fabricated using sample 416-90.

As shown in FIGS. 2-5, the initial brightnesses of the lamps are nearlyidentical. However, while the brightness of the lamps fabricated withthe uncoated phosphor decreased to half of their initial brightness inless than four hours of operation, the lamps fabricated with the coatedphosphor from 416-90 was operating above half brightness after more than200 hours of continuous operation. The maximum brightnesses measuredwith the uncoated and coated-phosphor lamps, the ratio of the maximumbrightnesses (coated versus uncoated) and the ratio of the half-lifevalues obtained with the uncoated and coated phosphor-containing lampsare all listed in Table 2, along with the comparable data for samples320-90, 327-90, and 403-90.

                                      TABLE 2                                     __________________________________________________________________________    Test Result Summary for Unpackaged EL Lamps Supported                         Upon ITO-Coated Glass                                                         Equivalent   Maximum Brightness (fl)                                                                   B(coated                                                                             t.sub.1/2 (coated)                            Sample No.                                                                          w/oAl(OH).sub.3                                                                      Uncoated                                                                            Coated                                                                              B(uncoated)                                                                          t.sub.1/2 (uncoated)                          __________________________________________________________________________    320-90                                                                              1.01   40.0  41.2  1.03   15                                            327-90                                                                              1.89   42.4  42.4  1.00   32                                            403-90                                                                              2.96   45.4  40.8  0.90   40                                            416-90                                                                              3.53   39.3  38.2  0.97   70                                            __________________________________________________________________________

As shown, the half-life of a coated-phosphor lamp, relative to that ofan uncoated-phosphor lamp, increases with increasing coating weighti.e., with increasing coating thickness. Sample 416-90 was also examinedusing the XPS surface analysis technique so as to gauge the extent towhich the ZnS:Cu phosphor particles are completely encapsulated by thecoating. Samples 320-90, 327-90, and 403-90, as well as the uncoatedphosphor (Lot 398), were similarly examined. The results of these fiveanalyses are summarized in Table 3 (where the surface concentrations ofZn, S, Al, 0, and Cu detected by the XPS technique are expressed on anatomic percent basis).

                  TABLE 3                                                         ______________________________________                                        XPS Surface Analysis Results for Uncoated and Coated                          Type 723 ZnS:Cu EL Phosphor                                                   Sample   Zn        S      Al      O    Cu                                     ______________________________________                                        Uncoated 33.35     39.13  n.d.    12.18                                                                              0.34                                   (Lot 398)                                                                     320-90   0.27      n.d.   42.21   55.95                                                                              n.d.                                   327-90   0.20      n.d.   40.89   53.22                                                                              n.d.                                   403-90   n.d.      n.d.   41.89   53.39                                                                              n.d.                                   416-90   0.04      n.d.   40.92   54.36                                                                              n.d.                                   ______________________________________                                    

As shown, no sulfur or copper was detected near the surface of anycoated material. Further, only a very small amount of zinc could bedetected with any of the coated materials. Thus, these materials appearto be virtually completely encapsulated in a coating composed of onlytwo elements detectable by XPS analysis: aluminum and oxygen.

EXAMPLE 2

Using three coated phosphor materials, Sample 320-90, 327-90 and 403-90along with the virgin (uncoated) phosphor, additional electroluminescentlamps were fabricated with sheets of flexible ITO-coated Mylar servingas the transparent support/transparent front electrode. In contrast toExample 1, these lamps were all heat-sealed between two sheets of 7.5mil thick water-impermeable Aclar. As before, each lamp was continuouslyoperated using a 120 V a.c. power supply driven at 400 Hz. The resultingbrightness and half-life values (obtained from the recorded brightnessversus time data for each lamp) are all listed in Table 4.

                                      TABLE 4                                     __________________________________________________________________________    Test Result Summary for Aclar-Packaged EL Lamps Supported Upon ITO-Coated     Mylar                                                                         Equivalent w/oAl(OH).sub.3                                                                      Maximum Brightness (fl)                                                                   B(coated)                                                                            t.sub.1/2 (coated)                       Sample No.                                                                          (Approx)    Uncoated                                                                            Coated                                                                              B(uncoated)                                                                          t.sub.1/2 (uncoated)                     __________________________________________________________________________    320-90                                                                              1           35.2  31.2  0.89   2.9                                      327-90                                                                              2           40.8  41.6  1.02   2.3                                      403-90                                                                              3.0         38.4  41.2  1.07   >2                                       __________________________________________________________________________

As shown in Table 4, the maximum brightnesses measured with the lampscontaining the three coated materials were all within approximately 10%of those measured with the corresponding uncoated-phosphor controllamps. Further, the thicker the coating, the brighter the coatedphosphor-containing lamps relative to the uncoated-phosphor controllamp. Most surprisingly, despite the fact that all of the lamps werewell sealed between sheets of the water-impermeable polymeric packagingmaterial, the half-lives of the lamps containing the coated phosphorswere all at least twice that of the corresponding uncoated-phosphorcontrol lamp.

The averaged brightness versus time data obtained with the lampscontaining coated phosphor number 320-90, along with the data obtainedwith the corresponding uncoated-phosphor control lamp, are plotted inFIG. 6. The corresponding data obtained with the lamps fabricated fromsample number 327-90 and its corresponding control lamp are similarlycompared in FIG. 7. The data for sample 403-90 are likewise compared inFIG. 8. In each case, the packaged coated phosphor-containing lamps ranconsiderably brighter than did the corresponding lamp containing thestandard uncoated phosphor. Moreover, the longer the lamps were burned,the greater the relative difference between the brightnesses of thelamps containing the coated and uncoated phosphors. This trend isparticularly noteworthy in the case of the lamps fabricated from sample403-90 (FIG. 8). In this case the coated phosphor-containing lampsyielded brightnesses 50% greater than those obtained with the uncoatedphosphor after only about 150 hours of continuous operation. These samelamps were about 75% brighter than the control lamp after burning forabout 300 hours.

EXAMPLE 3

In addition to the Aclar-packaged flexible lamps fabricated using theuncoated phosphor and coated phosphor materials described in Example 2,completely unpackaged i.e., unprotected, lamps were fabricated using theuncoated phosphor and coated phosphor number 403-90. These unpackagedlamps were burned alongside and at the same time as were thecorresponding Aclar-packaged lamps containing the uncoated and coatedphosphor. The average brightness versus time data obtained with thepackaged and unpackaged lamps fabricated from coated phosphor number403-90 are compared in FIG. 9. The corresponding brightness versus timedata obtained with the packaged and unpackaged lamps fabricated from thestandard, uncoated phosphor are similarly compared in FIG. 10.

As shown in FIG. 9, the completely unprotected lamps containing thecoated phosphor yielded brightnesses that were always within 10% ofthose obtained with the Aclar-packaged lamps. In contrast, thebrightnesses obtained with the unpackaged lamps fabricated from theuncoated phosphor decreased rapidly with time. As a result, theunpackaged uncoated phosphor-containing lamp brightnesses were, onaverage, only about 15% of those obtained with the Aclar-packaged lampcontaining the same uncoated phosphor. These results are illustrated inFIG. 11 in which the unpackaged-to-packaged lamp brightness ratiosobtained with both the uncoated and coated phosphors are compared.

Thus, whereas the severe moisture sensitivity of the uncoated phosphorcompletely prevents its use in unpackaged electroluminescent lamps, theapplication of the hydrolyzed TMA coatings formed via the fluidized bedCVD process outlined in this invention render the phosphor practicallymoisture insensitive. As a result, at least for some EL lampapplications, it appears that it may be possible to completely eliminatethe costly water-impermeable polymeric packaging materials that areabsolutely necessary in all EL lamps marketed today. On the other hand,the use of highly moisture-insensitive ZnS phosphors prepared accordingto the present invention used in conjunction with such water-impermeablepackaging materials perhaps also including thin layers of specialdesiccating substances to further prevent moisture from reaching theluminescent material, would result in EL lamps with lifetimes exceedingthose of any similar device produced today.

EXAMPLE 4

Electroluminescent lamps were fabricated upon sheets of ITO-coatedpolyester (Sierracin Intrex-100) and subsequently heat-sealed betweentwo sheets of either a relatively water-impermeable plastic (Aclar, aproduct of Allied Signal Corp.) or a relatively water-permeablepolyester material (obtained from General Binding Corp.). Identicallamps were fabricated using the as-received phosphor and the phosphorcoated following the reaction conditions outlined above in Table 1,Sample No. 508-90. The lamps were all evaluated using a 120 VAC powersupply operated at 400 Hz. The relative humidity in the test environmentwas maintained in the 50-60% range, and the ambient temperature rangedbetween 20° and 23° C. Lamp brightness was measured as a function oftime using a calibrated optometer.

The brightness versus time data obtained with the lamps fabricated withthe uncoated and coated phosphor (508-90) packaged in Aclar are comparedin FIG. 12. Whereas the initial brightness of the lamp containing thecoated phosphor was somewhat lower than that of the lamp containing theuncoated phosphor, equal brightnesses were measured after about 24 hoursof operation. Thereafter, the brightness of the lamp containing thecoated phosphor exceeded that of the lamp containing the uncoatedphosphor, the difference in brightness increasing with time ofoperation.

The comparable brightness versus time data obtained with the lampsfabricated with the uncoated and coated phosphor (508-90) packagedbetween sheets of the polyester material are similarly compared in FIG.13. Whereas the brightness of the coated phosphor lamp decreases verygradually over hundreds of hours of operation, the brightness of theuncoated phosphor lamp decreases very rapidly, dropping to about 25% ofits initial value within the first 24 hours of operation.

The brightness versus time data obtained with the uncoated-phosphorlamps packaged in Aclar and in the water-permeable polyester materialare compared in FIG. 14. The brightness versus time data obtained withthe coated-phosphor (508-90) lamps packaged in Aclar and in thepolyester material are similarly compared in FIG. 15. Whereas the coatedphosphor performs equally well in either packaging material, thestandard uncoated phosphor exhibits acceptable performance only in anessentially hermetically packaged lamp using a packaging material havingan extremely low water permeability.

EXAMPLE 5

This example compares the performance of two different lots of coatedphosphor in identically constructed electro-luminescent lamps. Twodifferent lots of Type 723 ZnS:Cu phosphor (lots 361S and 398S) wereemployed. As in Example 4, these two phosphor lots were coated using theconditions outlined in Table 1. The coated phosphors were assignedsample numbers 514-90 and 521-90, respectively. The EL lamps werefabricated upon sheets of the same ITO-coated polyester material. Lampscontaining each of the two coated-phosphor lots were packaged betweensheets of water-impermeable Aclar and between sheets of a relativelywater-permeable polyester material. As in previous Examples, the lampswere all tested using a 120 VAC/400 Hz power supply, the lamp brightnessbeing recorded as a function of operating time.

The brightness versus time data obtained with the two coated phosphorsin Aclar-packaged lamps are compared in FIG. 16. The data obtained withthe two coated phosphors packaged in polyester are likewise compared inFIG. 17. As shown, very similar lamp performance was measured with thetwo coated phosphors, independent of whether water-permeable orwater-impermeable packaging was used. Thus, these data demonstrate boththe moisture insensitivity of the coated phosphor and thereproducibility of the phosphor coating process itself.

EXAMPLE 6

A quantity of Type 723 ZnS:Cu phosphor (Lot RB361S) was fractionated bymeans of a 325 mesh sieve. The small-particle fraction was then coated,again following the coating conditions outlined in Table 1 for Samples508-90, 521-90 and 514-90. EL lamps were fabricated using thelarge-particle (>325 mesh) uncoated phosphor, the small-particle (<325mesh) uncoated phosphor, and the coated small-particle phosphors(612-90). Lamps containing each of the three phosphors were packaged inwater-impermeable Aclar and in a water-permeable polyester material. Allof the lamps were operated using a 120 VAC/400 Hz power supply.

The brightness versus time data recorded with the lamps packaged in thewater-permeable material are compared in FIG. 18. Whereas all threelamps had nearly identical initial brightnesses, the lamps containingthe large-particle and small-particle uncoated phosphor suffered veryrapid brightness reductions due to the extreme moisture sensitivity ofthe uncoated ZnS-based phosphor. In contrast, the lamp containing thecoated small-particle phosphor exhibited only a very gradual brightnessloss during the 200 hour measurement period, demonstrating the relativemoisture insensitivity of the coated phosphor.

The brightness versus time performance data obtained with the lampspackaged in the water-impermeable material are similarly compared inFIG. 19. In contrast to the polyester-packaged lamp data, all of thelamps exhibited relatively gradual reductions in brightness over the 600hour measurement period. However, the brightness of the lamp containingthe uncoated small-particle phosphor decreased more rapidly than didthat of either the lamp containing the uncoated large-particle phosphoror that containing the coated small-particle material. The poorermaintenance of the lamp containing the uncoated small-particle phosphoris consistent with earlier observations concerning the relationshipsbetween phosphor particle size and lamp performance. However, the factthat the coated small-particle phosphor exhibits a maintenance curvenearly identical to that of the uncoated large-particle phosphor is notanticipated by earlier results.

In general, the smaller the average particle size of the phosphor, themore uniform is the appearance of the electro-luminescent lamp (byvirtue of the greater particle density and improved thickness uniformityof the layer of phosphor particles which constitutes the basic lampelement). Thus, these data show that, by the use of coatedsmall-particle ZnS-based phosphors produced via the particle-coatingprocess described herein, an EL lamp manufacturer will be able tofabricate lamps of improved definition and uniformity with performancecharacteristics comparable to those typically obtained using thestandard, larger-particle uncoated phosphor.

The phosphor used in Examples 1-6 cited was copper activated zincsulfide EL phosphor. This phosphor was specifically developed for use inalternating current (A.C.) electroluminescent devices such as thosedescribed in the Examples. It is probable that all sulfide based ELphosphors suitable for use in A.C. EL devices may be made moistureinsensitive by application a hydrolyzed trimethyl aluminum coating. Itis also considered likely that sulfide-based EL phosphors developed foruse in direct current electroluminescent devices may be renderedmoisture insensitive by the application of the coatings described in thepresent invention. The following examples demonstrate the applicabilityof the process to other EL phosphors.

Examples 7, 8 and 9 described below demonstrate the effectiveness of thecoating process on three additional ZnS-based electroluminescentmaterials currently available from Sylvania. Example 10 demonstrates adifferent kind of moisture insensitivity and Example 11 attempts tofurther characterize the hydrolyzed TMA coatings. The coating reactionconditions for Examples 7-11 are listed below.

Phosphor Coating Conditions

Phosphor Weight: 300 gm

Carrier Gas: Purified N₂

Water Bubbler Temp./N₂ Flow Rate: 70° C./1.0 1/min.

TMA Bubbler Temp./N₂ Flow Rate: 30° C./0.5 1 min.

Coating Time/Temp.: 5 1/3 hr./200° C.

EXAMPLE 7

Electroluminescent lamps were fabricated upon sheets of ITO-coatedpolyester and subsequently heat-sealed between two sheets of arelatively water-permeable polyester material (obtained from GeneralBinding Corp.) The electro-luminescent phosphor used was Sylvania Type523 (Lot ELB357), a ZnS-based material co-doped with both copper andmanganese. Identical lamps were fabricated using the as-receivedphosphor and the phosphor coated following the reaction conditionsoutlined above (Sample No. 80390 with a 6.51% equivalent (Al(OH)₃loading). The lamps were evaluated using a 120 VAC power supply operatedat 400 Hz. The relative humidity in the test environment ranged between50 and 60%, and the ambient temperature ranged between 21 and 24° C.Lamp brightnesses were measured as a function of time using a calibratedoptometer.

The brightness versus time data obtained with the lamps fabricated withthe uncoated and coated phosphor are compared in FIG. 20. As shown, asomewhat higher initial brightness was measured with the lamp containingthe uncoated phosphor. However, after several hours of operation, thebrightness of the lamp containing the coated phosphor exceeded that ofthe lamp containing the uncoated phosphor, the difference in brightnessincreasing with time of operation. These data demonstrate the relativemoisture insensitivity of the coated manganese-doped phosphor and,conversely, the extreme moisture sensitivity of the standard, uncoatedphosphor.

EXAMPLE 8

This example demonstrates the effectiveness of the coating when appliedto another copper-doped ZnS-based electroluminescent material, Type 728ZnS:Cu, manufactured and marketed by Sylvania. Electroluminescent lampswere fabricated upon sheets of ITO-coated polyester and subsequentlyheat-sealed between two sheets of a relatively water-permeable polyestermaterial (obtained from General Binding Corp.). Two ZnS:Cuelectroluminescent phosphors were used: Sylvania Type 723 (Lot ELB398)and Sylvania Type 728 (Lot ELB418). Identical lamps were fabricatedusing the as-received phosphors and the phosphors coated following thereaction conditions outlined above. The coated version of Type 723,Sample 72090, contained an equivalent Al(OH)₃ loading of 5.70% while thecoated version of Type 728, Sample No. 80290, contained a 6.87%equivalent Al(OH)₃ loading. All of the lamps were evaluated using a 120VAC power supply operated at 400 Hz. Lamp brightnesses were measured asa function of time using a calibrated optometer.

The brightness versus time data obtained with the lamps fabricated withthe uncoated and coated phosphors are compared in FIG. 21. As shown,somewhat higher initial brightnesses were measured with the lampscontaining the uncoated phosphors. However, after several hours ofoperation, the brightnesses of the lamps containing both coatedphosphors exceeded those of the lamps containing both uncoatedphosphors. The difference in brightness between the coated and uncoatedphosphor-containing lamps increases with time of operation. Thus, bothcopper-doped phosphors Type 723 and Type 728 are renderedmoisture-insensitive via the application of a hydrolyzed TMA coating.Further, the brightnesses of non-hermetically packagedelectroluminescent lamps containing Sylvania Type 728 ZnS:Cu phosphorcoated with hydrolyzed TMA as described herein are substantially greaterthan are obtained with identically constructed and packaged lampscontaining Sylvania Type 723 ZnS:Cu phosphor similarly coated withhydrolyzed TMA.

EXAMPLE 9

This example demonstrates the effectiveness of the coating when appliedto another copper-doped ZnS-based electroluminescent material, Type 729ZnS:Cu, manufactured and marketed by Sylvania. Electroluminescent lampswere fabricated upon sheets of ITO-coated polyester using two ZnS:Cuelectroluminescent phosphors: Sylvania Type 728 (Lot ELB418) andSylvania Type 729 (Lot ELB396). Identical lamps were fabricated usingthe as-received phosphors and the phosphors coated following thereaction conditions outlined above. The coated versions of the Type 728and Type 729 phosphors, Samples 82290 and 82390, respectively, eachcontained an equivalent Al(OH)₃ loading of approximately 6.5%. The lampscontaining the as-received uncoated phosphors were heat-sealed betweentwo sheets of a relatively water-impermeable plastic (Aclar, a productof Allied Signal Corp.) The lamps containing the coated phosphors, onthe other hand, were packaged between sheets of a relativelywater-permeable polyester material (obtained from General Binding Corp.)All of the lamps were evaluated using a 120 VAC power supply operated at400 Hz. Lamp brightnesses were measured as a function of time using acalibrated optometer.

The brightness versus time data obtained with the lamps fabricated withthe uncoated and coated phosphors are compared in FIG. 22. As shown,somewhat higher initial brightnesses were measured with theAclar-packaged lamps containing the uncoated phosphors. However, afterseveral hours of operation, the brightnesses of the polyester-packagedlamps containing the coated phosphors exceeded those of the lampscontaining the uncoated phosphors. These data clearly demonstrate theremarkable moisture insensitivity of both phosphor types after theapplication of the hydrolyzed TMA coating.

EXAMPLE 10

This example demonstrates the EL-lamp performance of a ZnS-basedelectroluminescent material is unaffected by water washing either beforeor after the application of a hydrolyzed TMA coating formed as describedabove. Electro-luminescent lamps were fabricated upon sheets ofITO-coated polyester using Sylvania Type 729 ZnS:Cu EL phosphor (LotELB396). Identical lamps were fabricated using the uncoated phosphor,either as received or after a water wash followed by vacuum drying, andthe phosphor coated following the reaction conditions outlined above,either as-coated or after a water wash followed by vacuum drying. Thecoated version of the phosphor, Sample 82390, contained an equivalentAl(OH)₃ loading of approximately 6.5%. The lamps containing the uncoatedphosphor were heat-sealed between two sheets of a relativelywater-impermeable plastic (Aclar, a product of Allied Signal Corp.). Thelamps containing the coated phosphor, on the other hand, were packagedbetween sheets of a relatively water-permeable polyester material(obtained from General Binding Corp.). All of the lamps were evaluatedusing a 120 VAC power supply operated at 400 Hz. Lamp brightnesses weremeasured as a function of time using a calibrated optometer.

The brightness versus time data obtained with the untreated uncoatedphosphor in Aclar-packaged lamps and with the untreated coated phosphorin polyester-packaged lamps are compared in FIG. 23. The data obtainedwith the water-washed uncoated phosphor in Aclar packaged lamps and withthe water-washed coated phosphor in polyester-packaged lamps aresimilarly compared in FIG. 24. These data demonstrate not only that thecoated phosphor is insensitive to the effects of moisture in anoperating electroluminescent lamp, but that the effectiveness of thecoating is undiminished by immersion in water (followed by drying) priorto lamp fabrication.

EXAMPLE 11

The purpose of this example is to illustrate the fact that theproperties of the hydrolyzed TMA coating formed according to theteachings of this invention are substantially different from those thatwould be expected by one skilled in the chemical art. The reactionbetween TMA and water at temperatures no higher than a few hundreddegrees Centigrade should result in the formation of aluminum hydroxideand methane:

    Al(CH.sub.3).sub.3 +3CH.sub.4 +Al(OH).sub.3

The Al(OH)₃ produced by this low temperature reaction is expected to besubstantially amorphous. Electron diffraction analyses of a number ofZnS-based phosphors coated via the TMA hydrolysis reaction carried outin a gas-fluidized bed have revealed no evidence of crystallinity, inagreement with this expectation. However, XPS surface analyses ofseveral of the coated phosphor materials have yielded O/Al atomic ratiosof approximately 1.3, far from the 3.0 value expected for Al(OH)₃ butclose to the 1.5 value expected for Al₂ O₃ as shown in Table 3.

Thermogravimetric analyses have also been performed with a number ofcoated phosphor samples. Typical TGA data obtained with a sample ofuncoated Type 723 phosphor heated to above 800° C. at a 10° C./min. ramprate are shown in FIG. 25. As expected, the percent weight change versustemperature plot is essentially flat, indicating that the sample did notgain or lose weight significantly during the analysis. Comparable dataobtained with a sample of Type 723 phosphor coated via the TMAhydrolysis reaction (sample 40390 with a coating containing an amount ofaluminum equivalent to 3.0% Al(OH)₃) are similarly plotted in FIG. 26.In this case, an approximate 0.1% weight loss is detected occurring attemperatures between 350° and 600° C. Indeed, this is the temperaturerange over which amorphous Al(OH)₃ would be expected to transform to Al₂O₃ when so heated. However, for a material containing 3.0% Al(OH)₃, thecomplete conversion of the amorphous hydroxide coating to an oxide phaseby the elimination of water vapor would result in an approximate 1.0%weight loss, an order of magnitude greater than is observed. Similarresults have been obtained in a number of other TGA experiments withZnS-based materials coated via the TMA hydrolysis reaction. Thus, theTGA data suggest that the coating behaves more like an oxide than ahydroxide, contrary to our expectations.

Finally, there is the fact that the effectiveness of the coating formedat 200° C. is completely unaffected by water washing as described inExample 10 above. If the coating were an oxide formed at much highertemperatures, perhaps via the high temperature heat treatment of anamorphous hydroxide, this result would not be surprising. However, it issurprising indeed that a relatively thin amorphous aluminum hydroxidecoating formed at 200° C. should be apparently unaffected by thisprocedure.

Thus, whereas one skilled in chemistry would expect that a coatingformed via the reaction of TMA with water vapor at a temperature in thevicinity of 200° C. would consist essentially of amorphous aluminumhydroxide, all of the evidence accumulated to date conflicts with thatexpectation. Rather, the accumulated evidence suggests that the coatingso formed upon the surfaces of ZnS-based phosphor particles suspendedwithin a gas-fluidized bed (as described in this disclosure) consistssubstantially of some unidentified compound of aluminum and oxygen.

It is considered likely that coatings formed via a gas-phase reactionbetween TMA and H₂ O might also be effective in protecting so-calledthin-film EL devices from moisture attack. Such thin-film EL devicestypically contain layers of conducting, dielectric, and luminescentmaterials that may be formed via gas-phase reactions not at all unlikethe TMA/H₂ O reaction used to form the moisture-protective barriersdescribed above. Thus, at least in principle, it would be relativelysimple to incorporate such TMA/H₂ O reactions into existingmanufacturing processes to form one or more thin moisture-protectivebarrier layers that become integral parts of such EL devices. The TMA/H₂O reaction carried out at a temperature between 100° C. and 300° C.might be used to coat the thin polycrystalline phosphor film beforeapplying the final insulating and conductive electrode layers. It mayeven be possible to entirely eliminate the insulating layer from suchdevices since, to the best of our knowledge, the coatings formed via theTMA/H₂ O reaction are themselves electrically insulating.

While there has been shown and described what are at present consideredthe preferred embodiments of the invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the scope of the invention as defined bythe appended claims, specifically the use of other alkylaluminums suchas triethylaluminum or the use of a stirred-bed rather than afluidized-bed reactor.

What is claimed is:
 1. A method for forming a continuous coating of onphosphor particles comprising:a) vaporizing trimethyl aluminum in acarrier gas to form a carrier gas containing vaporized trimethylaluminum; b) vaporizing water in a carrier gas to form a carrier gascontaining vaporized water; c) passing said carrier gas containingvaporized trimethyl aluminum and said carrier gas containing vaporizedwater through a finely divided phosphor powder in which phosphorparticles are suspended in the carrier gases and to envelop thefluidized particles in said carrier gas containing vaporized trimethylaluminum and said carrier gas containing vaporized water; and d)reacting the vaporized trimethyl aluminum and the vaporized water on theparticles of the phosphor powder to form a continuous coating comprisinghydrolyzed trimethyl aluminum of predetermined thickness on the phosphorparticles.
 2. The method according to claim 1 wherein the fluidized bedis maintained at a temperature of less than 300° C.
 3. The methodaccording to claim 1 wherein the phosphor powder comprises zinc sulfide.4. A method for forming a continuous coating on particles of a finelydivided material comprising:a) vaporizing an aluminum-containingmaterial selected from the group consisting of trimethylaluminum andtriethylaluminum in an inert carrier gas to form a gas containing avaporized aluminum-containing material; b) vaporizing water in a carriergas to form a carrier gas containing vaporized water; c) passing saidcarrier gas containing vaporized aluminum-containing material and saidcarrier gas containing vaporized water through a finely divided materialin which particles of the finely divided material are suspended in thecarrier gases wherein the aluminum-containing material reacts with thewater on surfaces of the finely divided material to form a continuouscoating comprising hydrolyzed trimethyl aluminum or hydrolyzed triethylaluminum of predetermined thickness on the finely divided material. 5.The method according to claim 4 wherein the finely divided materialcomprises phosphor particles.
 6. The method according to claim 5 whereinthe phosphor particles comprise copper activated zinc sulfide.
 7. Themethod according to claim 5 wherein the fluidized bed is maintained at atemperature of less than 300° C.